1. Field of the Invention
This invention relates to a radio communication method and a radio communication system between a base station and a mobile station which are applied to a digital cordless telephone system which makes use of a personal handyphone system (PHS) and by which communication by a modem modulation signal such as a facsimile modem signal or a data modem signal or a speech signal is performed by demultiplexing of a PCM (Pulse Code Modulation) signal of 64 kbps and an ADPCM (Adaptive Differential PCM) signal of 32 kbps.
2. Description of the Related Art
A digital cordless telephone system which makes use of a handyphone system (PHS) is conventionally used. An exemplary one of such digital cordless telephone systems is disclosed, for example, in Japanese Patent Laid-Open Application No. Heisei 5-328425.
The cordless telephone system disclosed in the document mentioned above is directed to realization of talking of a high quality by an ADPCM signal of 32 kbps. Further, as a multiplexing system for a speech signal, a circuit having a transmission speed of 64 kbps such as an ISDN (Integrated Services Digital Network) is used. In the multiplexing system, an ADPCM signal of 32 kbps is multiplexed to allow duplex communication by 32 kbps.
In this instance, a mutual exchanging apparatus between a .mu.-law, A-law system of 64 kbps as a digitalization sampling system for a speech signal and an ADPCM system of 32 kbps is provided to allow setting of which one of the systems should be used. By such mutual exchanging processing, if, during communication using the ADPCM system of 32 kbps, a new request for communication to the same communication destination is received, additional communication using the same circuit is possible.
However, in the conventional cordless telephone system described above which is directed to realization of talking of a high quality by an ADPCM signal of 32 kbps, when a base station connected to a public network performs data transmission through a radio circuit regarding a modem modulation signal from a facsimile (FAX) modem or a data modem as a speech signal, a PHS mobile station which receives the ADPCM signal of 32 kbps by the transmission converts the ADPCM signal of 32 kbps having a narrow bandwidth once into an analog signal and sends out the analog signal to the facsimile machine or data terminal.
Consequently, the data quality is deteriorated considerably and besides the transmission speed is dropped. In this instance, the transmission speed of 9,600 bps is guaranteed for practical use. This also applies to another case wherein a base station is connected to an ISDN network.